The present invention relates to a tubular casing, to a process for its manufacture and to its use.
Tubular casings made from a web, for example, of regenerated cellulose, which are manufactured by folding the web along its longitudinal axis and glueing the overlapping edge zones running along the longitudinal axis are well known. See, e.g., U.S. Pat. Nos. 2,148,884, 2,226,442, 2,685,769, 2,685,770 and 2,757,495. The glueing operations hitherto described, however, comprise either the use of solvents by means of which the surface of the web is partly dissolved and is tackified, whereupon an "adhesive" is produced in situ on the surface of the web, or the use of webs which are coated with a thermoplastic resin or consist of a thermoplastic material which can be sealed when heat and pressure are applied. See, e.g., U.S. Pat. No. 2,773,773. Acrylates which can be tackified by application of heat or by means of a solvent are also mentioned as examples of suitable resins in U.S. Pat. No. 2,653,432. Furthermore, it has also been proposed heretofore to use an acid solution of a water-soluble, partially polymerized thermosetting resin as the adhesive agent, with polymerization of the resin being accelerated by the presence of an acid (pH 4.5-5.5 ) in the aqueous solution. In U.S. Pat. No. 2,607,696, as examples of such resins, there are mentioned aminoplast resins, such as urea/formaldehyde or melamine/formaldehyde resins, ketone/formaldehyde resins and phenolic resins which can be hardened at 180.degree. to 220.degree. C. and thereby glue the overlapping edge regions which are pressed together. U.S. Pat. No. 2,686,725 describes a tubular casing of the foregoing type in which the thermosetting resin, for example, a melamine/formaldehyde resin, is applied, in acid solution to the still moist fiberfree cellulose gel which has not yet been dried, and a heat-sealing layer of polyethylene resin in the form of a tape or as a powder is melted onto the subsequently hardened resin. In order to bond the overlapping edge regions of the cellulose hydrate web, the polyethylene is melted by the application of pressure and elevated temperature, whereupon the overlapping edges are heat-sealed to one another.
U.S. Pat. No. 2,653,432 similarly describes a process for bonding the overlapping edges with a strip of thermoplastic material which becomes tacky when heated.
In contrast, the sausage casings of German Offenlegungsschrift No. 1,908,851 consist of a film of plastic, and the longitudinal seam is produced by heat-sealing the overlapping edges. A tear-tape, which can comprise a polyester carrier film with an adhesive coating, is sealed onto the overlapping region. Application of the tear-tape to the outer side of the tubular casing is also known according to U.S. Pat. No. 3,545,161.
Insofar as these tubular casings actually consist of naturally occurring polymers, they can all be manufactured only in a relatively troublesome manner, that is to say, at high temperatures, with long residence times until the adhesive has set and with the aid of acids or organic solvents which can be removed from the tubular casing only by difficult and very expensive measures. Furthermore, they are not sufficiently gas-and smoke permeable in the glueing region. For these reasons, tubular casings of naturally occurring polymers, for example, based on cellulose, have for many years been predominantly manufactured without a seam, although, because of the relatively slow web speed, this tubular spinning process is very cost-intensive and permits only a limited tube diameter. In addition, it is not entirely possible to achieve a uniform tube diameter and wall thickness, and it is very complicated and expensive to carry out finishing processes, for example, coating the inside of the tubing. In addition, in the field of tubular casings of naturally occurring polymers which are provided with a longitudinal seam, in recent years there have been hardly any publications which give an indication as to how the problems mentioned above could be overcome.
Furthermore, these known tubular casings are not suitable for the manufacture of curved sausages.
For certain types of sausages, the synthetic sausage casings are manufactured from naturally occurring material, in particular based on cellulose, e.g. regenerated cellulose, in a curved or rounded circular shape, i.e., the so-called wreath-shaped casings. The curvature has hitherto been produced by stretching the tube on one side during its manufacture. For example, in one of these processes (U.S. Pat. No. 2,136,566), the seamless shells of coagulated and subsequently regenerated cellulose hydrate gel which have been manufactured in a known manner from viscose are blown up with air, before drying, and wound around a heated cylinder. As a result, one side shrinks, i.e., the side of the spirally wound seamless casing which comes into contact with the hot surface of the cylinder. By this means a certain degree of stability is imparted to the spiral-shaped casing. Drying of the spiral-shaped casing is then brought to completion in a tunnel oven. Spirally wound tubular casings for the manufacture of sausages wound in circles are obtained in this manner.
In another known process for the manufacture of seamless wreath-shaped casings (U.S. Pat. No. 2,925,621), a viscose solution is extruded through an annular-slot die into a spinning bath which exerts a coagulating effect on the viscose. After the required degree of stability has been achieved, the coagulated but not yet completely regenerated tube obtained in this manner is blown up with air or an inert gas and, in the blown-up state, is wound around a cylinder once or several times in a spiral-like manner. This spirally wound tube of coagulated viscose is converted into regenerated cellulose, for example, by immersion in a regenerating bath. Finally, the seamless tube of regenerated cellulose is subjected to the customary purification, after-treatment and drying measures.
Thus, these known curved sausage casings are also manufactured by the tubular spinning process and exhibit the abovementioned general disadvantages of tubes manufactured without seams.
Moreover, it is also known to manufacture seamless wreath-shaped casings by partial shrinkage in the longitudinal direction under the action of heat (U.S. Pat. No. 3,679,435). However, these tubes consist entirely of a synthetic thermoplastic polymer and furthermore are seamless.